Magnetic read head structure with improved bottom shield design for better reader performance

ABSTRACT

A bottom shield in a read head is modified by including a non-magnetic decoupling layer and second magnetic layer on a conventional first magnetic layer. The second magnetic layer has a magnetization that is not exchange coupled to the first magnetic layer, and a domain structure that is not directly affected by stray fields due to domain wall motion in the first magnetic layer. Accordingly, the modified bottom shield reduces shield related noise on the reader and will provide improved signal to noise (SNR) ratio and better reader stability. The second magnetic layer may be further stabilized with one or both of an antiferromagnetic coupling scheme, and insertion of an antiferromagnetic pinning layer. In dual readers, the modified bottom shield is used in either the bottom or top reader although in the latter, first magnetic layer thickness is reduced to maintain reader-to-reader spacing and acceptable bit error rate (BER).

This is a divisional application of U.S. patent application Ser. No.16/007,014; filed on Jun. 13, 2018, which is herein incorporated byreference in its entirety, and assigned to a common assignee.

RELATED PATENT APPLICATION

This application is related to the following: U.S. Pat. No. 9,230,577;assigned to a common assignee and herein incorporated by reference inits entirety.

TECHNICAL FIELD

The present disclosure relates to an improved bottom shield design forstabilizing free layer magnetization in a single reader or dual reader(2DMR) scheme, and in particular to a bottom shield stack comprised of anon-magnetic decoupling layer, and an uppermost magnetic layer withenhanced domain stability that is formed between a bottommost magneticlayer in the bottom shield and a bottom surface of a sensor in thereader structure.

BACKGROUND

In a magnetic recording device in which a read head comprises amagnetoresistive (MR) sensor, there is a constant drive to increaserecording density. One trend used in the industry to achieve thisobjective is to decrease the size of the MR sensor. Typically, thesensor stack has two ferromagnetic layers that are separated by anon-magnetic layer. One of the ferromagnetic layers is a reference orpinned layer wherein the magnetization direction is fixed by exchangecoupling with an adjacent antiferromagnetic (AFM) pinning layer. Thesecond ferromagnetic layer is a free layer with a magnetization thatrotates in response to external magnetic fields, and is rotated towardseither parallel or anti-parallel to the magnetization in the pinnedlayer to read out the local orientation of magnetic moment in therecording media. When passing the MR sensor over a recording medium atan air bearing surface (ABS), the free layer magnetic moment will rotateaccording to the local magnetic field generated by the recording media.By processing the angle of rotation as a function of location on themedia, the data pattern recorded on the media can be decoded. A MRsensor may be based on a tunneling magnetoresistive effect where the twoferromagnetic layers are separated by a thin non-magnetic dielectriclayer. A sense current is used to detect a resistance value, which islowest when the moments from the two layers are parallel to each otherand is the highest when the two moments are anti-parallel to each other.In a current perpendicular-to-plane (CPP) configuration, a sense currentis passed from a top shield through the sensor layers to a bottomshield, or in the reverse direction.

A longitudinal biasing scheme is typically used in a read head design tokeep the free layer in a stable orientation in the absence of theexternal magnetic field. Bias films of high coercivity or soft bias alsoknown as junction shields, are abutted against the edges of the MRsensor and particularly against the sides of the free layer. Byarranging for the flux flow in the free layer to be equal to the fluxflow in the adjoining hard bias layer, the demagnetizing field at thejunction edges of the aforementioned layers vanishes because of theabsence of magnetic poles at the junction. As the critical dimensionsfor MR sensor elements become smaller with higher recording densityrequirements, the free layer becomes more volatile and more difficult tobias. Top and bottom magnetic shields with in-plane magnetization areoften used to ensure the MR sensor will only respond to a local magneticfield. However, free layer magnetization is sensitive to domain wallmotion in the bottom and top shield, which may lead to increased noise,reducing the signal to noise (SNR) ratio of the reader sensor and causefailure in decoding data from the media.

In recent years, 2DMR configurations have become attractive from anareal density improvement standpoint. However, shield stability is moredifficult to control in 2DMR schemes because of a requirement to shrinkreader-to-reader spacing (RRS), and in view of repeated thermaltreatments during recording head fabrication that can readily change themagnetization orientation in the shields. Shield instability willdirectly translate into reader instability and will adversely impact SNRand bit error rate (BER). A new read head structure is needed whereinshield stability is improved while maintaining acceptable SNR and BER.

SUMMARY

One objective of the present disclosure is to provide a bottom shielddesign that enables improved stability in the shield and also to haveimproved free layer magnetization in a single reader, or in one or bothreaders in a 2DMR structure.

A second objective of the present disclosure is to provide a method offorming the bottom shield structure according to the first objective.

According to one embodiment of the present disclosure where a readercomprises a MR sensor formed between a bottom shield and a top shield,the bottom shield structure has a stack of layers wherein a non-magneticdecoupling layer and a second magnetic layer with a magnetizationsaturation (Ms) value from 5 kiloGauss (kG) to 15 kG are sequentiallyformed on a first magnetic layer. The first magnetic layer has athickness greater than the overlying decoupling layer and secondmagnetic layer, and also has a Ms value from 5 kG to 15 kG. Thedecoupling layer is made of a conductive material such as one or more ofCu, Al, W, Cr, Ta, Ru, Pt, and Pd, or is an insulating materialincluding but not limited to a metal oxide that is Al₂O₃, SiO₂, MgO,Ta₂O₅, and TiO₂, and has a thickness that is sufficient to preventmagnetic coupling between the first and second magnetic layers.

The second magnetic layer is comprised of one or more of FeCo, FeCoNi,FeCoN, NiFe, NiFeW, NiFeCr, NiFeMo, or alloys thereof such as FeCoRwhere R is Mo, Zr, Nb, Hf, Ru, Pt, Re, Pd, or a combination thereof, andhas a thickness from about 1 nm to 1 micron. In some embodiments, thesecond magnetic layer is a single layer with an in-plane magnetizationaligned in the same direction as overlying junction shields on each sideof a free layer in the MR sensor. In an alternative embodiment, thesecond magnetic layer is a trilayer in which an antiferromagnetic (AF)coupling layer is sandwiched between two ferromagnetic layers (FM1 andFM2) in a FM1/AF coupling layer/FM2 configuration to stabilize themagnetization. In yet another embodiment, there may be anantiferromagnetic (AFM) layer inserted within the second magnetic layerto yield a FM1/AFM/FM2 configuration, or inserted between the decouplinglayer (DL) and second magnetic (FM) layer to give a DL/AFM/FM orDL/AFM/FM1/AF coupling layer/FM2 stack of layers on the first magneticlayer. In some embodiments, the second magnetic layer may be comprisedof amorphous materials such as CoTaZr to provide a smoother surface onwhich to fabricate the MR sensor. In preferred embodiments, the secondmagnetic layer has a fine grain structure to improve domain stabilitytherein.

In one embodiment, the bottom shield structure of the present disclosureis formed in a single reader. According to a second embodiment, thebottom shield structure disclosed herein is employed in one or both of abottom reader and a top reader in a 2DMR scheme.

The present disclosure also includes a process involving a sequence ofsteps to form the bottom shield including a physical vapor deposition(PVD) of the decoupling layer on the first magnetic layer, and thenelectroplating the second magnetic layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a head arm assembly of the presentdisclosure.

FIG. 2 is side view of a head stack assembly of the present disclosure.

FIG. 3 is a plan view of a magnetic recording apparatus of the presentdisclosure.

FIG. 4 is a down-track cross-sectional view of a combined read-writehead wherein a single reader is formed between top and bottom shields inthe read head portion according to an embodiment of the presentdisclosure.

FIG. 5 is an ABS view of a prior art reader structure wherein a bottomshield is a single magnetic layer that is used to shield the reader frommagnetic fields on the bottom shield side away from the sensor.

FIG. 6 is an ABS view of a 2DMR structure previously fabricated by theinventors wherein a first bottom shield and second bottom shield forshielding magnetic fields away from the sensor on the bottom shield sidein a first reader and second reader, respectively.

FIG. 7 is a top-down view of the reader in FIG. 5 showing the relativesize of the reader and bottom shield, and a possible magnetizationpattern within the bottom shield.

FIG. 8 is an ABS view of read head structure wherein a bottom shield iscomprised of a non-magnetic decoupling layer and a second magnetic layersequentially formed on a first magnetic layer according to an embodimentof the present disclosure.

FIG. 9 is a top-down view of the second magnetic layer in FIG. 8 thathas a width and height essentially equivalent to the width and height ofthe underlying first magnetic layer according to one embodiment of thepresent disclosure.

FIG. 10 is a top-down view of the second magnetic layer in FIG. 8 havinga width and height less than the width and height of the underlyingfirst magnetic layer according to another embodiment of the presentdisclosure.

FIG. 11 is an ABS view of an alternative embodiment where the secondmagnetic layer in FIG. 8 has an AF coupling layer between twoferromagnetic (FM) layers to stabilize a magnetization direction in theupper FM layer.

FIG. 12 is an ABS view of another embodiment of the present disclosurewherein the second magnetic layer in FIG. 8 is modified to include anAFM layer.

FIG. 13 is an ABS view of another embodiment of the present disclosurewherein the second magnetic layer in FIG. 11 is modified to include anAFM layer that adjoins a top surface of the decoupling layer.

FIG. 14 is an ABS view of a 2DMR structure wherein a bottom shield inthe bottom reader is comprised of a non-magnetic decoupling layer and asecond magnetic layer sequentially formed on a first magnetic layeraccording to an embodiment of the present disclosure.

FIG. 15 is an ABS view of a 2DMR structure wherein a bottom shield inthe top reader is comprised of a non-magnetic decoupling layer and asecond magnetic layer sequentially formed on a first magnetic layeraccording to another embodiment of the present disclosure.

FIGS. 16A-16B are plots comparing magnetic performance in terms ofcumulative distribution of Barkhausen jump normalized by sensoramplitude for readers with and without the modified bottom shieldstructure of the present disclosure.

FIGS. 17A-17B are plots comparing magnetic performance in terms ofcumulative distribution of hysteresis normalized by sensor amplitude forreaders with and without the modified bottom shield structure of thepresent disclosure.

DETAILED DESCRIPTION

The present disclosure is a bottom shield design that provides improvedmagnetic stabilization in a read head that is a single reader or has a2DMR configuration. The more stable bottom shield ensures there is lessstray field that could act on the sensor element either directly orindirectly via magnetic coupling to other magnetic parts of the reader.Improved bottom shield stability translates into improved MR sensor(free layer) stability, as well as better SNR and BER. In the drawings,the y-axis is in a cross-track direction, the z-axis is in thedown-track direction, and the x-axis is in a direction orthogonal to theABS and towards a back end of the read head. The stabilized bottomshield design described herein is not limited to a particular read headstructure, and is effective even in a reader in a multiple readerstructure such as 3DMR. The terms “second” and “secondary” may be usedinterchangeably when referring to the upper layer in the bottom shieldstack of layers. In addition, the terms “read head” and “reader” areused interchangeably.

Referring to FIG. 1, a HGA 100 includes a magnetic recording head 101comprised of a slider and a combined read-write structure formedthereon, and a suspension 103 that elastically supports the magneticrecording head. The suspension has a plate spring-like load beam 222formed with stainless steel, a flexure 104 provided at one end portionof the load beam, and a base plate 224 provided at the other end portionof the load beam. The slider portion of the magnetic recording head isjoined to the flexure, which gives an appropriate degree of freedom tothe magnetic recording head. A gimbal part (not shown) for maintaining aposture of the magnetic recording head at a steady level is provided ina portion of the flexure to which the slider is mounted.

HGA 100 is mounted on an arm 230 formed in the head arm assembly 103.The arm moves the magnetic recording head 101 in the cross-trackdirection y of the magnetic recording medium 140. One end of the arm ismounted on base plate 224. A coil 231 that is a portion of a voice coilmotor is mounted on the other end of the arm. A bearing part 233 isprovided in the intermediate portion of arm 230. The arm is rotatablysupported using a shaft 234 mounted to the bearing part 233. The arm 230and the voice coil motor that drives the arm configure an actuator.

Next, a side view of a head stack assembly (FIG. 2) and a plan view of amagnetic recording apparatus (FIG. 3) wherein the magnetic recordinghead 101 is incorporated are depicted. The head stack assembly 250 is amember to which a first HGA 100-1 and second HGA 100-2 are mounted toarms 230-1, 230-2, respectively, on carriage 251. A HGA is mounted oneach arm at intervals so as to be aligned in the perpendicular direction(orthogonal to magnetic medium 140). The coil portion 253 of the voicecoil motor is mounted at the opposite side of each arm in carriage 251.The voice coil motor has a permanent magnet 263 arranged at an oppositeposition across the coil 253.

With reference to FIG. 3, the head stack assembly 250 is incorporated ina magnetic recording apparatus 260. The magnetic recording apparatus hasa plurality of magnetic media 140 mounted to a spindle motor 261. Forevery magnetic recording medium, there are two magnetic recording headsarranged opposite one another across the magnetic recording medium. Thehead stack assembly and actuator except for the magnetic recording heads101 correspond to a positioning device, and support the magneticrecording heads, and position the magnetic recording heads relative tothe magnetic recording medium. The magnetic recording heads are moved ina cross-track of the magnetic recording medium by the actuator. Themagnetic recording head records information into the magnetic recordingmedia with a perpendicular magnetic recording (PMR) writer element (notshown) and reproduces the information recorded in the magnetic recordingmedia by a magnetoresistive (MR) sensor element (not shown).

Referring to FIG. 4, magnetic recording head 101 comprises a combinedread-write head previously fabricated by the inventors and disclosed inrelated U.S. Pat. No. 9,230,577. The down-track cross-sectional view istaken along a plane formed orthogonal to the ABS 30-30, and that bisectsthe main pole layer 14. The read head is formed on a substrate 80 thatmay be comprised of AlTiC (alumina+TiC) with an overlying dielectriclayer 81 that is made of a dielectric layer such as alumina. Thesubstrate is typically part of a slider formed in an array of sliders ona wafer. After the combined read head/write head is fabricated, thewafer is sliced to form rows of sliders. Each row is typically lapped toafford an ABS before dicing to fabricate individual sliders that areused in a magnetic recording device. A bottom shield 1 is formed ondielectric layer 81.

A magnetoresistive (MR) element also known as MR sensor 2 is formed onbottom shield 1 at the ABS 30-30 and typically includes a plurality oflayers that are described later with regard to FIG. 4. A top magneticshield 15 is formed on the MR sensor. Layer 82 adjoins the backside ofthe MR sensor, and layer 83 contacts the backsides of the bottom shieldand top shield. An insulation layer 8 and a top shield (S2B) layer 9 aresequentially formed on the top shield. Note that the S2B layer 9 mayserve as a flux return path (RTP) in the write head portion of thecombined read/write head. Thus, the portion of the combined read/writehead structure formed below layer 9 in FIG. 4 is typically considered asthe read head.

The present disclosure anticipates that various configurations of awrite head (PMR writer) may be employed with the read head portion. Inthe exemplary embodiment, magnetic flux 70 in main pole layer 14 isgenerated with flowing a current through bucking coil 60 b and drivingcoil 60 d that are below and above the main pole layer, respectively,and are connected by interconnect 61. Magnetic flux 70 exits the mainpole layer at pole tip 14 p at the ABS 30-30 and is used to write aplurality of bits on magnetic media 140. Magnetic flux 70 b returns tothe main pole through a trailing loop comprised of trailing shields 17,18, PP3 shield 26, and top yoke 18 x. Layer 41 adjoins top yoke 18 x.There is also a leading return loop for magnetic flux 70 a that includesleading shield 11, leading shield connector (LSC) 33, S2 shieldconnector (S2C) 32, return path 9, and back gap connection (BGC) 62. Themagnetic core may also comprise a bottom yoke 35 below the main polelayer. Dielectric layers 10, 13, 19, 22, 37-39, 43, and 45 are employedas insulation layers around magnetic and electrical components. Aprotection layer 27 covers the PP3 trailing shield and is made of aninsulating material such as alumina. Above the protection layer andrecessed a certain distance u from the ABS 30-30 is an optional coverlayer 29 that is preferably comprised of a low coefficient of thermalexpansion (CTE) material such as SiC. Overcoat layer 28 is formed as theuppermost layer in the write head.

Referring to FIG. 5, an ABS view is shown of a portion of the read headincluding bottom shield 1, top shield 15, junction shields 21, and theMR sensor having a lower layer 2 d, middle free layer 2 f, and upperlayer 2 h that is formed on a center section of the bottom shield.Sidewalls 2 s connect a bottom surface 2 b with the top surface 2 t ofthe MR sensor. There is a non-magnetic isolation layer 40 formed alongthe sidewalls 2 s and on portions of the bottom shield that are notcovered by the MR sensor. Magnetization 21 m in the junction shields isprimarily responsible for providing longitudinal biasing to the freelayer. Magnetization 15 m in the top shield stabilizes the magnetizationdirection in the junction shields through exchange coupling. Layer 2 hcomprises at least a capping layer, and layer 2 d includes a referencelayer with a fixed magnetization direction, and a non-magnetic spacer(not shown) between the reference layer and free layer 2 f. Layer 2 dmay also include a bottommost seed layer, and an antiferromagnetic (AFM)layer such as IrMn or another Mn alloy may be formed on a side of thereference layer that faces away from the free layer to pin themagnetization direction in the reference layer. Shield-to-shield spacingis depicted as RSS1. In other embodiments, the AFM layer (not shown) maybe recessed behind the MR sensor stack or embedded in a back portion ofbottom shield 1 to satisfy reduced RSS1 requirements. The non-magneticspacer may be comprised of one or more metal oxides, metal oxynitrides,or metal nitrides to provide a tunneling magnetoresistive effect.

A conventional 2DMR structure is shown in FIG. 6 wherein a first readersuch as the reader structure in FIG. 5 is used as a bottom reader. Abovefirst top shield 15 is an isolation (dielectric) layer 25 thatmagnetically and electrically isolates the first reader from a second(top) reader that includes a second MR sensor 3 comprised of bottomlayer 3 d, free layer 3 f, and upper layer 3 h where layers 3 d, 3 f,and 3 h may have the same composition and function as layers 2 d, 2 f,and 2 h, respectively. The second MR sensor is formed between a secondbottom shield 50 with magnetization 50 m and a second top shield 55. Asecond insulation layer 48 adjoins the sidewalls of the second MRsensor. Junction shields 31 formed on the second insulation layer have amagnetization 31 m and are employed to provide a longitudinal bias tothe free layer 3 f magnetization direction. Magnetization 31 m is in thesame direction as magnetization 55 m in the second top shield because offerromagnetic coupling. Preferably, the second MR sensor is alignedabove the first MR sensor such that center plane 44 bisects each of theaforementioned MR sensors. The center plane is orthogonal to the ABS.Here, reader-reader spacing RRS is depicted as the down-track distancefrom the center of the first reader to the center of the second reader.

Referring to FIG. 7, a top-down view of MR sensor 2 on bottom shield 1is shown with overlying layers removed. The MR sensor has width a, whichis substantially smaller width than width w of the bottom shield, andheight b substantially less than height h of the bottom shield.Moreover, the magnetic domains 1 a-1 d in the bottom shield may form aclosed loop such that a first domain 1 a proximate to front side 1 f atthe ABS 30-30 has a cross-track direction for magnetization 1 m, and aback portion with domain 1 c proximate to backside 1 e has amagnetization 1 m anti-parallel to that in the first domain. Sidedomains 1 b, 1 d adjacent to sides 1 s 1, 1 s 2, respectively, haveanti-parallel magnetizations that are orthogonal to the ABS in order toform a closed loop for magnetization 1 m in a counterclockwisedirection.

According to a first embodiment of the present disclosure depicted inFIG. 8, the bottom shield in a single reader is modified to have atrilayer stack 1 x wherein first magnetic (bottom shield) layer 1 isretained from the prior art, and a non-magnetic decoupling layer 4 witha thickness t1 from 1 nm to 0.5 micron, and a second magnetic layer 5with a thickness t2 from 1 nm to 0.5 micron are sequentially formedthereon.

The first magnetic layer 1 has a thickness t greater than 0.5 micron,and preferably substantially greater than the second magnetic layer 5.The decoupling layer 4 is made of a conductive material such as one ormore of Cu, Al, W, Cr, Ta, Ru, Pt, and Pd, or is an insulating materialincluding but not limited to a metal oxide that is Al₂O₃, SiO₂, MgO,Ta₂O₅, or TiO₂, and has a thickness that is sufficient to preventexchange coupling between the first magnetic layer and second magneticlayer.

Both of the first magnetic layer 1 and second magnetic layer 5 have a Msvalue from 5 kG to 15 kG. Each of the first and second magnetic layersare comprised of one or more of FeCo, FeCoNi, FeCoN, NiFe, NiFeW,NiFeCr, NiFeMo, or alloys thereof such as FeCoR where R is Mo, Zr, Nb,Hf, Ru, Pt, Re, Pd, or a combination thereof. Preferably, a material isselected for the second magnetic layer that provides a stable domain ina cross-track direction (magnetization 5 m) even in the presence ofexternal magnetic fields, and is not susceptible to domain wall motionproximate to the sensor that leads to instability. Moreover, a materialwith a fine grain structure or amorphous character such as CoTaZr isbeneficial in forming a smooth top surface 5 t on which to build a MRsensor with better film uniformity, which in turn yields improvedperformance. Top shield 15, and junction shields 21 are generallycomprised of CoFe, CoFeNi, CoFeN, or NiFe, or a combination thereof. Insome embodiments, one or both of the top shield and junction shields maybe comprised of stacks of magnetic materials separated by one or morenon-magnetic materials (such as Ru, Cr, Rh, Ir, Mo, Re, and Os) that canprovide antiferromagnetic coupling between the adjacent magnetic layers.

In the exemplary embodiment shown in FIG. 8, the second magnetic layer 5is a single layer with in-plane magnetization 5 m aligned in the samecross-track direction as magnetization 21 m in overlying junctionshields, and as magnetization 2 m in free layer 2 f. Magnetization 15 mis ferromagnetically coupled to magnetization 21 m that provides a biasfield to stabilize free layer magnetization. Because of decoupling layer4, domain wall motion that tends to form in the first magnetic layer 1has no destabilization effect on magnetization in the second magneticlayer. Note that RSS1 is maintained compared with the prior art singlereader in FIG. 5. However, a total thickness of bottom shield 1 x isincreased by (t1+t2) compared with the bottom shield in FIG. 5 if thethickness of the first magnetic layer is constant.

An important feature is that the first magnetic layer 1 is a greaterdown-track distance from free layer 2 f in the embodiment shown in FIG.8 than in the prior art MR sensor in FIG. 5. Accordingly, stray fieldsthat may arise due to domain wall motion in the first magnetic layerhave a significantly smaller effect on free layer magnetization in viewof the greater separation from the MR sensor in FIG. 8.

Referring to FIG. 9, a top-down view of the second magnetic layer 5 andMR sensor 2 is shown with overlying layers removed according to anembodiment of the present disclosure. In a preferred embodiment, theshape of the second magnetic layer including the width w between side 5s 1 and side 5 s 2 and height h between a front side 5 f at the ABS30-30 and a backside 5 e is optimized independent of the width andheight of the underlying first magnetic layer for better shieldstability.

As an example, in an alternative embodiment depicted in FIG. 10, heighth1 and width w1 of the second magnetic layer 5 are less than height hand width w, respectively, of the first magnetic layer 1. However,height h1 is preferably at least 5× greater than height b of the MRsensor, and width w1 is preferably at least 5× greater than width a ofthe MR sensor in order to prevent stray fields (from domain wall motion)in the first magnetic layer from disrupting magnetization in the freelayer.

According to another embodiment shown in FIG. 11, there is a modifiedbottom shield 1 x-1 wherein a second magnetic layer 5-1 has a trilayerstack 5 a/5 b/5 c in which bottom ferromagnetic (FM) layer 5 a and topFM layer 5 c are anti-ferromagnetically (AF) coupled through middle AFcoupling layer 5 b. Thus, magnetization 5 m 1 in FM layer 5 c is alignedparallel to magnetization 21 m in the overlying junction shields, and isstabilized through AF coupling with FM layer 5 a that has magnetization5 m 2 anti-parallel to magnetization 5 m 1. AF coupling layer 5 b istypically Ru, but may also be one of Rh, RhRu, Mo, Re, Os, or Ir. FMlayer 5 a is decoupled from magnetization (not shown) in first magneticlayer 1 because of the intermediate decoupling layer 4. Thus, bottomshield 1 x-1 has a 1/4/5-1 stack of layers.

In another embodiment illustrated in FIG. 12, the bottom shield ismodified with the insertion of AFM layer 6 between the decoupling layer4 and bottom FM layer 5 a in the second magnetic layer 5-1 to give abottom shield 1 x-2 with a 1/4/6/5-1 stack of layers. AFM layer 6 isadvantageously used to pin magnetization 5 m 2 in FM layer 5 a, which inturn further stabilizes magnetization 5 m 1 in FM layer 5 c because ofAF coupling between FM layers 5 a, 5 c. The AFM layer is preferably oneof IrMn, PtMn, PdMn, NiMn, OsMn, RuMn, RhMn, RuRhMn, or MnPtPd.

In yet another embodiment shown in FIG. 13, the bottom shield ismodified with the insertion of AFM layer 6 within the second magneticlayer to give a configuration 5-2 having a 5 a/6/5 c stack of layerswhere bottom FM layer 5 a and top FM layer 5 c are pinned and aligned inthe same cross-track direction through contact with AFM layer 6. Theresulting bottom shield 1 x-3 has a 1/4/5 a/6/5 a stack of layers. Inthis case, magnetization 5 m 1 is stabilized because of FM layer 5 ccontact with the AFM layer 6 (also known as the AFM pinning layer)rather than indirectly through antiferromagnetic coupling with FM layer5 a as in the previous embodiment. Alternatively, layer 5 a may be madeof a non-magnetic material.

The present disclosure also encompasses an embodiment wherein the bottomshield in a 2DMR configuration has enhanced stabilization because ofincorporating one of the previously described bottom shield designs 1 x,1 x-1, 1 x-2, or 1 x-3. For example, FIG. 14 depicts a 2DMR designwherein the reader with MR sensor 2 and bottom shield 1 x described inFIG. 8 serves as the bottom reader. All layers including MR sensor 3 inthe top reader are retained from the 2DMR structure shown in FIG. 6.Only the bottom shield in the bottom reader is modified to provideadditional stability to junction shield magnetization 21 m and to freelayer magnetization 2 m. Furthermore, RRS is maintained from thepreviously described 2DMR structure.

Referring to FIG. 15, the present disclosure anticipates an embodimentwhere the bottom shield in the second read head (top reader) isstabilized with a modified bottom shield 50 x having a 50/51/52 stack oflayers where decoupling layer 51 has the same properties andcomposition, and serves the same function as decoupling layer 4 inprevious embodiments, and second magnetic layer 52 with magnetization 52m is essentially equivalent in composition and function to previouslydescribed second magnetic layer 5. In embodiments where thickness t4 offirst magnetic layer 50 in bottom shield 50 x is equal to thickness t3of the first magnetic layer (bottom shield) in the previous 2DMRembodiment, BER may be degraded because RRS2>RRS. However, in preferredembodiments, thickness t4 is reduced so that RRS2 (FIG. 15)=RRS (FIG.14), without significantly compromising BER in the second reader whileenabling improved stabilization of magnetization 31 m in junctionshields 31, and of magnetization 3 m in free layer 3 f. In allembodiments described herein, one or both of first magnetic layer 1 andfirst magnetic layer 50 continue to protect the overlying MR sensor fromstray (external) magnetic fields.

Although not shown, the present disclosure anticipates that both bottomshields 1, 50 in the 2DMR structure shown in FIG. 6 may be replaced witha modified bottom shield to provide enhanced stability to free layermagnetization 2 m and free layer magnetization 3 m, respectively. Inother words, the 2DMR configuration shown in FIG. 14 may be modifiedwith the replacement of bottom shield 50 with bottom shield 50 x fromthe FIG. 15 embodiment.

The present disclosure also encompasses a method of fabricating amodified bottom shield 1 x, 1 x-1, 1 x-2, or 1 x-3 described previously.Note that the process flow for forming a MR sensor with junction shieldson a bottom shield is found in related U.S. Pat. No. 9,230,577. Withregard to FIG. 8, the first magnetic layer 1 is formed on a substratesuch as dielectric layer 81 (FIG. 4) by an electroplating method, forexample, to give a thickness t from 0.5 to 5 microns. Thereafter, thedecoupling layer 4 is preferably deposited at the desired thickness t1with a physical vapor deposition (PVD) process at a temperature from 25°C. to 250° C. Second magnetic layer may be electroplated on thedecoupling layer, or optionally deposited by a PVD method, depending onthe thickness t2, composition, and desired film uniformity. In someembodiments, bottom shield layers 4 and 5 may be formed in the samechamber with two sequential PVD steps without breaking a vacuum tominimize process time.

To demonstrate the benefits of the modified bottom shield describedherein, an experiment was performed to compare a MR sensor stabilizedusing a conventional bottom shield with a MR sensor that is stabilizedwith a bottom shield 50 x described previously with respect to FIG. 15.In particular, a 2DMR structure was fabricated with a top sensor in asecond reader (R2) formed on a bottom shield having a trilayer(50/51/52) stack of layers according to an embodiment of the presentdisclosure, and a bottom MR sensor in a first reader (R1) formed on aconventional bottom shield consisting of a 2 micron thick NiFe layer. Aplurality of 2DMR devices was formed on two different wafers beforebeing probed with a quasi static tester where a resistance across a MRsensor is measured while scanning with an external magnetic field.

FIGS. 16A-16B are plots showing the cumulative distribution ofBarkhausen jump normalized by sensor amplitude when an external magneticfield is scanned from −600 Oe to +600 Oe. Threshold is defined as theratio of Barkhausen jump to the sensor amplitude. The plots are showingthe percentage of heads that show jump values less than the threshold.In both wafer 1 (FIG. 16A) and wafer 2 (FIG. 16B), the second reader(top reader) clearly has significantly better distribution than thefirst reader (bottom reader). Note that the cumulative distributionencompasses results from a plurality of MR sensor devices across eachwafer.

FIGS. 17A-17B are plots showing the cumulative distribution ofhysteresis as a function of sensor amplitude when an external magneticfield is scanned from −600 Oe to +600 Oe. Here, threshold is defined asthe cumulative distribution of hysteresis as a function of sensoramplitude. Again, for both wafer 1 (FIG. 17A) and wafer 2 (FIG. 17B), R2has significantly better performance than R1. Accordingly, R2 isexpected to have SNR that is substantially improved over SNR for R1.More importantly, R2 will have better stability after excitation by amagnetic field.

In summary, we have disclosed a scheme for improved stabilization in aMR sensor wherein a bottom shield has improved stability because of amultilayer stack wherein an uppermost magnetic layer adjoining junctionshields in the MR sensor is stabilized compared with a conventionalsingle layer bottom shield because of decoupling from a bottommostmagnetic layer that is prone to domain wall motion (instability).Moreover, in some embodiments additional stability is provided throughAF coupling between the upper magnetic layer and a middle magnetic layerin the multilayer stack, and through insertion of an AFM pinning layerin the multilayer stack. When the improved bottom shield design isincorporated in one or both of a first reader (bottom reader) and asecond reader in a 2DMR structure, or in a single reader structure, anenhanced sensor performance is realized in terms of better signal tonoise ratio, which helps to improve bit error rate (BER) performance.Moreover, improved sensor stability is also realized because of theimproved shield stability.

While this disclosure has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this disclosure.

I claim:
 1. A magnetic read head structure, comprising: (a) a firstmagnetoresistive (MR) sensor formed on a first bottom shield and with afront side at an air bearing surface (ABS), and wherein the first MRsensor has a first free layer with a first magnetization in a firstcross-track direction, and sidewalls that extend from a top surfacethereof to the first bottom shield; (b) the first bottom shield,comprising: (1) a bottommost magnetic layer having a first thickness(t), a width (w) between two sides thereof, and a height (h) between afront side at the ABS and a backside; (2) an uppermost magnetic layerthat is a single layer and has a second magnetization in the firstcross-track direction, a second thickness that is substantially lessthan the first thickness, and having a width w1 and a height h1, whereinw1<w, and h1<h, and w1 and h1 are at least 5× greater than a width and aheight, respectively, of the first MR sensor; (3) a middle magneticlayer; and (4) a non-magnetic decoupling layer that adjoins a topsurface of the bottommost magnetic layer, and is below a bottom surfaceof the middle magnetic layer; and (c) a first junction shield that isadjacent to the first MR sensor sidewalls, and having a thirdmagnetization in the cross-track direction that provides a bias field inthe cross-track direction to stabilize the first magnetization in thefree layer.
 2. The magnetic read head structure of claim 1 wherein thefirst MR sensor is part of a combined read/write head.
 3. The magneticread head structure of claim 1 wherein the non-magnetic decoupling layerhas a thickness from about 1 nm to 0.5 micron and is comprised of aconductive material that is one or more of Cu, Al, W, Cr, Ta, Ru, Pt,and Pd, or is an insulating material that is Al₂O₃, SiO₂, MgO, Ta₂O₅, orTiO₂.
 4. The magnetic read head structure of claim 1 wherein theuppermost magnetic layer in the bottom shield has a magnetic saturation(Ms) value from 5 kiloGauss (kG) to 15 kG, and is one or more of FeCo,FeCoNi, FeCoN, NiFe, NiFeW, NiFeCr, NiFeMo, or a FeCoR alloy where R isone or more of Mo, Zr, Nb, Hf, Ru, Pt, Re, and Pd.
 5. The magnetic readhead structure of claim 4 wherein the uppermost magnetic layer has athickness from about 1 nm to 0.5 micron.
 6. The magnetic read headstructure of claim 1 wherein the bottom shield is further comprised ofan antiferromagnetic (AF) coupling layer between the middle anduppermost magnetic layers that enables a magnetization in the middlemagnetic layer to be anti-parallel to the second magnetization in theuppermost magnetic layer.
 7. The magnetic read head structure of claim 1wherein the bottom shield is further comprised of an antiferromagnetic(AFM) pinning layer formed between the middle magnetic layer and thedecoupling layer thereby pinning a magnetization in the middle magneticlayer.
 8. The magnetic read head structure of claim 1 wherein the bottomshield further comprises an antiferromagnetic (AFM) pinning layer thatadjoins a bottom surface of the uppermost magnetic layer, and contacts atop surface of the middle magnetic layer, wherein the secondmagnetization in the uppermost magnetic layer is pinned because of theAFM pinning layer.
 9. The magnetic read head structure of claim 1further comprising; (a) a first top shield formed on the first MRsensor; (b) an isolation (dielectric) layer formed on a top surface ofthe first top shield; (c) a second bottom shield formed on the isolationlayer; (d) a second MR sensor formed on the second bottom shield andhaving a second free layer with a fourth magnetization in thecross-track direction, the second MR sensor has sidewalls that extendfrom a top surface thereof to the second bottom shield; and (e) a secondjunction shield that is adjacent to the second MR sensor sidewalls, andhaving a fifth magnetization in the cross-track direction that providesa bias field in the cross-track direction on the second free layer. 10.A head gimbal assembly (HGA), comprising: (a) the read head structure ofclaim 1; and (b) a suspension that elastically supports the magneticread head structure, wherein the suspension has a flexure to which themagnetic read head structure is joined, a load beam with one endconnected to the flexure, and a base plate connected to the other end ofthe load beam.
 11. A magnetic recording apparatus, comprising: (a) theHGA of claim 10; (b) a magnetic recording medium positioned opposite toa slider on which the magnetic read head structure is formed; (c) aspindle motor that rotates and drives the magnetic recording medium; and(d) a device that supports the slider, and that positions the sliderrelative to the magnetic recording medium.